1. Field of the Invention
The present invention relates to a technique of selecting codes suitable for constant amplitude multiplexing and preventing extra information from being added for constant amplitude multiplexing through a combination of the selected codes to thereby prevent a reduction in transmission efficiency due to multiplexing in a CDM (Code Division Multiplexing) communication system using a frequency spreading scheme to improve a transfer rate in a limited frequency band.
2. Description of the Related Art
Conventionally, in order to adjust a multi-level signal generated by adding several orthogonal codes such that it has a constant amplitude of a uniform amplitude, a method of limiting a signal to have a certain level by using a level clipping has been used. This method is simple but has shortcomings in that the orthogonality of the orthogonal codes may be damaged, thereby bringing about information loss. Thus, this method can be applicable to voice or multimedia communications insensitive to an error without causing a problem, while being unsuitable for data communications sensitive to a loss of data therein.
In addition, a method for modulating data by selecting one of orthogonal codes allocated by blocks by using transmission data has been proposed. This method, however, is disadvantageous in that it cannot implement various spread coefficients.
Also, a method for performing constant amplitude conversion by using a Hadamard matrix has been also proposed. In this method, however, when an orthogonal code having a large spread coefficient, a repeated operation must be performed by layers in order to increase the spread coefficient, and in this case, in order to prevent modulation signals generated when orthogonal codes outputted from respective layers are added from becoming multi-level signals, extra information must be included. Thus, for example, if a spread coefficient is 16 at every spreading, only nine chips of user data is included over a 16-chip output, obtaining an efficiency of 9/16. Namely, this method has reduced transmission efficiency.